Devices are needed that can detect and characterize neutron and ionizing electromagnetic radiation sources in large objects at large distances. Such devices are of great interest for detecting and monitoring the movement of fissile and other nuclear materials. Gamma detectors and helium monitors are used at transportation portals for monitoring people, vehicles and large trucks.
In these applications, large area detectors are necessary for the collection of low source strength radiations within reasonable detection times. What is a reasonable time may be dictated by the economic costs associated with the slowing down of personnel and goods.
A viable alternative, in terms of radiation detection, is the employment of many detectors to cover the same area. Many detectors, however, carry the burden of associated electronics, cables, and processing electronics.
Fast plastic scintillation detectors are favored for use in portal monitors because of their sensitivity to gamma rays, and also because they can be made in large sizes inexpensively. The fast plastic output light can be used to measure time correlations of neutrons and gamma rays that characterize the fission chain multiplication processes in plutonium, uranium or other materials. The time dependent coincidence counts between pairs of detectors can and has been used in the past to characterize fissile material.
Another device that is applicable to the radiation detection problem is the 6Li loaded glass fiber detector where 6Li loaded glass fibers are embedded in polyethylene. The polyethylene moderates the fast neutrons and allows their subsequent absorption by the 6Li.
In the conventional 6Li detector, the 6Li loaded glass fibers are embedded in a polyethylene slab that can be up to one meter long. Since the glass fibers detect thermal neutrons preferentially, fast neutrons from fission must be slowed to low energy to be detected by the 6Li (n γ) reaction. To achieve this, additional polyethylene of various thicknesses is placed in front and back of the fiber panels to form a thermal neutron detector. The light produced in the fibers is collected at both ends and viewed by photomultiplier tubes. Electrical pulses produced by the photomultiplier tubes can be counted in coincidence or not. Coincidence counting eliminates photomultiplier and other electronic noise contributions.
The 6Li fiber optic detectors are designed for high-efficiency thermal neutron detection, and their utility for thermal coincidence counting and multiplicity counting is known. The time constants (˜100 ns) of the fission chain multiplication and decay processes of fissile material cannot be observed with the 6Li fiber optic detectors due to the polyethylene moderator (10's of microseconds) used in the detector. The time distribution of coincident counts only includes information about the moderator, although the amplitude is related to the quantity of fissile material.
The present invention is a unified layer structure merging a 6Li loaded glass scintillation fiber-optic thermal neutron detector with a fast scintillation detector. Detection of thermal and fast neutrons and ionizing electromagnetic radiation is achieved in a single detector structure.